Current collector for organic electrolyte batteries

ABSTRACT

Corrosion of the current collectors in cells utilizing Vanadium Oxides and Chromates as cathode materials is prevented by the utilization of metals from Groups IVb, Vb and VIb of the Periodic Tables for fabricating the current collectors and leads in contact with the cathode materials. Ta, Ti, Mo and W, are preferred materials. Cells containing such structures are described as well as an intermediate protective structure.

FIELD OF THE INVENTION

This invention relates to high energy density organic electrolyte cells based upon light metal anodes and strongly oxidizing cathodes and more particularly to specifically useful structures and structural materials for cathodes utilizing vanadium oxides and metal chromates or dichromates.

BACKGROUND OF THE INVENTION

The Li/V₂ O₅ (Ser. No. 829,849 now abandoned) and Li/metal chromate (U.S. Pat. No. 3,658,592) organic electrolyte cells show excellent energy density and storability at ambient temperatures. However, at elevated temperature (e.g. 55°C or higher) the cells exhibit corrosion of the nickel cathode current collector.

1. The Li/V₂ O₅ organic electrolyte cells made according to the procedure described in Ser. No. 829,849, showed severe corrosion of the nickel current collector and the nickel tab to the degree that the tab was disconnected from the current collector after a storage of three months at 55°C. The corrosion was particularly severe at the point where the nickel tab was welded onto the expanded nickel current collector. This resulted in a dead cell due to the lack of electrical contact between the cathode terminal and the cathode current collector.

OBJECTS OF THE INVENTION

a. To solve the problem of corrosion of cathode collector, and

B. TO IMPROVE THE ELEVATED TEMPERATURE STORABILITY OF ORGANIC ELECTROLYTE BATTERIES.

THE INVENTION

I have discovered that it is possible to protect nickel current collector tabs for cathode assemblies to permit an extension of the corrosion resistant life of cathodes beyond the three month storage, 55°C storage, maximum life expectancy of unprotected nickel current collectors. My method for protecting the current collector tabs in the weld areas which are most prone to corrosion consists in heat sealing layers of polyolefinic polymer foils on both sides of the tab weld. The heat-sealed foil extends completely around the tab-weld area and thus prevents the access and contact of electrolyte and cathode active material to the corrosion-prone tab-weld area.

The details of this aspect of my invention are illustrated in the drawing wherein:

FIG. 1 shows the cathode assembly current collecting grid exposed in the area of the tab-weld;

FIG. 2 shows the cathode assembly of FIG. 1 with the nickel tab welded in place to the current collector;

FIG. 3 shows the cathode tab-weld area of FIG. 2 protected by heat sealed polyethylene foils; and

FIG. 4 is an enlarged view along line 4 . . . 4 of FIG. 3 showing the tab-weld area detailing the protection of the tab-weld area by the polyethylene foils heat sealed to the grid and embedded in the cathode active material. The procedure according to this aspect of the invention resulted in cells which survived the accelerated test of three months of storage with the electrolyte in contact with the cathode.

However, I have found that cathodes so constructed, showed tab and current collector corrosion after 6 months of storage at 55°C showing that the problem while successfully ameliorated still persisted.

My original solution to the corrosion problem was directed to the tab-weld area of the current collector assembly where the nickel current collector embedded in the cathode active material was fastened to the nickel tab leading from the cathode to the cell terminal connections. It had been noted that this was the focus of the corrosion noted when the cells were stored under accelerated aging conditions.

Welding conditions were varied, corrosive resistant alloys were used for the tabs, the welded areas as well as the current collectors and tabs were plated with protective yet highly conductive metals such as gold, but none of these expedients imparted sufficient corrosion protection to warrant the effort and expense. Then I discovered that by protecting only the most corrosion-prone area immediate, the tab-weld area, from access by the electrolyte by heat sealing electrolyte proof foils around these critical areas, I could make cells that uniformly survived storage at 55°C for 3 months. Few of the previous expedients survived this test and none did so uniformly. The heat-sealed foils of this invention are foils of polyolefin polymer materials such as polyethylene and polypropylene.

Heat-sealable films of such materials are commercially available in various thicknesses ranging from 0.001 to 0.010 inch in thickness. I have found 0.002 inch thickness of polyethylene to be satisfactory and preferred as it is easily heat sealed around the tab-weld area. Such seals uniformly and adequately protect this critical corrosion prone area. The heat seals to the metal are easily fabricated with light, localized pressure by heated platens.

The current collectors of this aspect of this invention are fabricated by welding the nickel tabs 4, to Exmet grids 3. Then the areas of the weld 5 are overlain on each side of the weld by 2 mil polyethylene foils 6 and heat sealed by platens at the temperatures recommended for the specific brand of foils.

After this protection treatment, the cathode-active material 2 is applied to the current collector assembly by the techniques described in U.S. Pat. No. 3,658,592 and copending Ser. No. 829,849 to form cathode 1.

A variant method for applying the cathode active material 2 from the method described in U.S. Pat. No. 3,658,592 is that the chromate cathodes are dried at 50°C in a vacuum instead of at 300°C at atmospheric pressure as previously described. 60°

I have further discovered that metals of Group IVb, Vb, VIb of the Periodic Table do not corrode when in contact with cathodic oxidizing agents such as V₂ O₅, HgCrO₄ and Ag₂ CrO₄, even at elevated temperatures.

Further, I have discovered that cathode collectors of Tantalum (Ta), Titanium (Ti), Molybdenum (Mo), Zirconium (Zr), Niobium (Nb), Vanadium (V), Chromium (Cr) and Tungsten (W) in contact with such cathode-active materials are useful in the manufacture of cathodes for electrochemical cells based upon lithium or other light active metal anodes, organic electrolytes and such cathodes.

Such cells have extended shelf life when stored at 55°C beyond comparable cells manufactured with such conventional "anti-corrosive" current collectors as nickel, silver, stainless steel, Inconel, gold-plated nickel and polyolefin protected nickel.

It is a theory that the unique corrosion resistant properties of the metals of Group IVb, Vb and VIb of the Periodic Table particularly Ta, Ti, Mo, and W, which are preferred, are derived from their property of forming protective oxide films on contact with these oxidants in the presence of the organic electrolytes. These oxide films while protective of the underlying metal possess a high degree of conductivity. Such cells show no increase in internal resistance when compared to cells using conventional current collectors.

DETAILS OF THE INVENTION

The V₂ O₅, HgCrO₄ and Ag₂ CrO₄ cathodes, made in a manner similar to that described in the copending application Ser. No. 829,849 and U.S. Pat. No. 3,658,592, on the expanded-metal substrates such as Ni, Inconel, gold plated Ni, Ag, Ta, Ti, Mo, Zr, Nb, V, Cr, W and stainless steel, were refluxed at 85°C in an electrolyte consisting of 1M LiClO₄ in an equivolume mixture of tetrahydrofuran and propylene carbonate, in order to accelerate the corrosion phenomenon. The reflux temperature was used to accelerate the corrosion test.

At the end of 233 hours of refluxing, the cathodes were examined. It was found that the expanded metal grids of Ta, Zr, Nb, V, Cr, W and Ti, Mo did not show any signs of corrosion, whereas all the other metals were partially or completely corroded.

Of the protected non-corroding metal grids and tabs, thereof Tantalum, Titanium and Molybdenum are preferred because of the ease of manufacture, lower cost and commercial availability.

The elevated temperature stability of the Li/V₂ O₅, Li/HgCrO₄ and Li/Ag₂ CrO₄ organic electrolyte cells were thus improved by the use of cathode grids and tabs made of metals such as Ta, Ti and Mo. The metals such as Zr, Nb, V, Cr and W also improve the elevated temperature storability of the cells when used for cathode grids and tabs.

These cathode current collector materials may be used in the form of expanded metal sheets, wires, flat tabs, and cathode cups. The active material may be packed around the exmet, may have screens of wire of these metals included therein or the sheets may be fashioned into terminal tabs, or cathode cups.

These current collectors while particularly effective for Vanadium Oxide and metal chromate and dichromate cathodes in organic electrolytes may also be used for milder cathode active materials such as other metal oxides, halides, permanganates, arsenates, periodates, persulfates and sulfites. The metal chromates include dichromates as well as chromates of Ag, Hg, and Cu.

The organic electrolyte includes combinations of electrolyte salts and solvents including electrolyte salts such as perchlorate, hexafluorophosphate, tetrafluoroborate, and hexafluoroarsenate of lithium.

Solvents such as propylene carbonate, tetrahydrofuran, dimethoxy ethane, methyl formate, acetonitrile, dimethyl carbonate, dimethyl sulfoxide, dimethyl sulfite, dimethyl formamide, gamma-butyrolactone, and N-nitrosodimethylamine and mixtures thereof are included.

Anodes for such cells include Li, Na, K, Ca, Mg, and Al. 

What is claimed is:
 1. A cathode current collector assembly for use under corrosion-prone conditions which comprises a current collector body and a terminal tab welded thereto wherein only the immediate area where the tab is welded to said current collector is overlain, surrounded by and has heatsealed thereto foils consisting of polyolefin polymer.
 2. The assembly according to claim 1 wherein said polymer is polyethylene. 